Harvest the Rain
(Page 3 of 8)
August/September 2003
Adapted from Environmental Building News
But if you're in an area that experiences frequent drought conditions, or in an area with sporadic rainfall and a decidedly dry season, plan accordingly. Experts recommend planning for half your region's expected rainfall, while projecting twice as much water use.
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SIZING YOUR SYSTEM
First, calculate your water usage. For residential systems, this includes toilet flushing, bathing, clothes washing, dishwashing and outdoor watering. With water-conserving plumbing fixtures and little or no outside watering, per-person usage can range from 55 to 75 gallons per day. If you're already connected to municipal water, study your monthly water bills to get your average household usage.
Next, determine rainwater availability in your area. A quick estimate of rainwater collection can be made based on your region's annual rainfall, but more thorough calculations will examine average, minimum and maximum rainfall on a per month basis. This information should be available from the National Weather Service or a local meteorological station. In areas with less than 24 inches of rainfall per year, building a large enough collection area and cistern to provide adequate water may not be feasible.
Actual collection calculations are made based on the available roof area (the projected horizontal area of the roof surface used for collection) and an "efficiency coefficient," which accounts for the fact that not all the rainwater falling on the roof gets into the cistern. (See "Calculating Catchment Capability," below.)
Size the cistern to provide enough water storage for reasonably expected dry periods. The Texas Guide to Rainwater Harvesting includes detailed information on carrying out all of these calculations, including an advanced method that factors in storage capacity.
CATCHMENT, IF YOU CAN
The most common rainwater catchment system is a roof. Typically, this is the roof of the building where the water will be used, though it may be a separate building designed expressly for rainwater harvesting (a "water barn"). Nearly all types of roofs have been — and are being — used for rainwater collection, but some are better than others.
The best roofing material for rainwater catchment is uncoated stainless steel or factory-enameled galvanized steel with a baked-enamel, certified lead-free finish. With any metal coating, ask the manufacturer whether the coating contains heavy metals (red paint used on metal often contained lead in the past). Any existing metal roof being used for a potable water catchment system should be tested for lead.
Wood shakes, concrete or clay tiles, and asphalt shingles are more likely than other materials to support the growth of mold, algae, bacteria and moss, which can potentially contaminate water supplies. Treated wood shingles may leach toxic preservatives, and asphalt shingles may leach small amounts of petroleum compounds. In addition to the health concerns, a porous or rough roof surface holds back some of the water that would otherwise make it into the cistern. Asphalt roofing has a "collection efficiency" of about 85 percent while enameled steel has a collection efficiency of more than 95 percent. With asphalt roofing, more of the rainwater stays on the roof in a typical rainstorm (i.e., the roof stays wet), though the actual percentage will depend on the duration of the storm.
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